LME49810 Datasheet, PDF (13/21 Pages) National Semiconductor (TI) – 200V Audio Power Amplifier Driver with Baker Clamp

LME49810
www.ti.com
SNAS391C – MAY 2007 – REVISED APRIL 2013
Once the maximum package power dissipation has been calculated using Equation (2), the maximum thermal
resistance, θ_SA, (heat sink to ambient) in °C/W for a heat sink can be calculated. This calculation is made using
Equation (3) which is derived by solving for θ_SA from Equation (2).
θ_SA = [(T_JMAX−T_AMB)−P_DMAX(θ_JC +θ_CS)] / P_DMAX
(3)
Again it must be noted that the value of θ_SA is dependent upon the system designer's amplifier requirements. If
the ambient temperature that the audio amplifier is to be working under is higher than 25°C, then the thermal
resistance for the heat sink, given all other things are equal, will need to be smaller.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet the design targets of an application. The choice of
external component values that will affect gain and low frequency response are discussed below.
The overall gain of the amplifier is set by resistors R_F and R_i for the non-inverting configuration shown in
Figure 1. The gain is found by Equation (4) below given R_i = R_IN and R_F = R_S.
A_V = R_F / R_i (V/V)
(4)
For best Noise performance, lower values of resistors are used. A value of 243 is commonly used for R_i and
setting the value for R_F for desired gain. For the LME49810 the gain should be set no lower than 10V/V. Gain
settings below 10V/V may experience instability.
The combination of R_i and C_i (see Figure 1) creates a high pass filter. The gain at low frequency and therefore
the response is determined by these components. The -3dB point can be determined from Equation (5) shown
below:
f_i = 1 / (2πR_iC_i) (Hz)
(5)
If an input coupling capacitor (C_IN) is used to block DC from the inputs as shown in Figure 1, there will be
another high pass filter created with the combination of C_IN and R_IN. The resulting -3dB frequency response due
to the combination of C_IN and R_IN can be found from Equation (6) shown below:
f_IN = 1 / (2πR_INC_IN) (Hz)
(6)
For best audio performance, the input capacitor should not be used. Without the input capacitor, any DC bias
from the source will be transferred to the load. The feedback capacitor (C_i) is used to set the gain at DC to unity.
Because a large value is required for a low frequency -3dB point, the capacitor is an electrolytic type. An
additional small value, high quality film capacitor may be used in parallel with the feedback resistor to improve
high frequency sonic performance. If DC offset in the output stage is acceptable without the feedback capacitor,
it may be removed but DC gain will now be equal to AC gain.
COMPENSATION CAPACITOR
The compensation capacitor (C_C) is one of the most critical external components in value, placement and type.
The capacitor should be placed close to the LME49810 and a silver mica type will give good performance. The
value of the capacitor will affect slew rate and stability. The highest slew rate is possible while also maintaining
stability through out the power and frequency range of operation results in the best audio performance. The value
shown in Figure 1 should be considered a starting value with optimization done on the bench and in listening
testing. Please refer to Slew Rate vs. C_C Graph in TYPICAL PERFORMANCE CHARACTERISTICS for
determining the proper slew rate for your particular application.
SUPPLY BYPASSING
The LME49810 has excellent power supply rejection and does not require a regulated supply. However, to
eliminate possible oscillations all op amps and power op amps should have their supply leads bypassed with low-
inductance capacitors having short leads and located close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency
instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large electrolytic capacitor
(10μF or larger) which is used to absorb low frequency variations and a small ceramic capacitor (0.1μF) to
prevent any high frequency feedback through the power supply lines. If adequate bypassing is not provided the
current in the supply leads which is a rectified component of the load current may be fed back into internal
circuitry. This signal causes low distortion at high frequencies requiring that the supplies be bypassed at the
package terminals with an electrolytic capacitor of 470μF or more.
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